The invention relates to improved single-site transition metal catalysts useful for the polymerization of xcex1-olefins wherein the active catalyst species is formed in-situ, i.e., in the polymerization reaction system. For preparation of the catalysts of this invention, a transition metal precatalyst is prepared by contacting a transition metal complex containing at least one labile ligand capable of being removed and replaced with an alkyl group, an ionizing agent and, optionally, a support material, and thereafter contacting the precatalyst with an organometallic alkylating agent in the polymerization reaction system. By producing the catalyst in this manner it is possible to eliminate instability problems heretofore observed with transition metal complexes which have alkyl substituents associated with the transition metal. Furthermore, with certain types of transition metal complexes, significant improvements in catalytic activity and/or improvements in resin properties can be achieved when the catalysts are produced in situ in the manner described herein.
Metallocene catalyst systems comprised of a metallocene compound and aluminoxane cocatalyst are known and have good activity for the polymerization of xcex1-olefins. While the resulting polymers and copolymers typically have acceptable molecular weight and molecular weight distribution and good morphology, aluminoxanes are relatively expensive and must be used at high levels relative to the transition metal. Ratios of 500 to 1000 moles Al per mole of transition metal are not uncommon. These high levels of cocatalyst increase manufacturing costs and can also result in unacceptable levels of aluminum residue in the resulting resin.
To overcome the problems associated with the use of aluminoxane cocatalyzed metallocene catalysts, catalyst systems have been developed based on cationic metallocene catalysts formed using cocatalysts capable of forming a stable anion. Ionic organoboron compounds are commonly used as cocatalysts in these systems. While the cationic metallocene catalysts eliminate the use of aluminoxanes, there still are certain disadvantages associated with their use. First and foremost is the need to use alkyl-substituted transition metal complexes to obtain an active cationic catalyst species and the recognized instability of such alkyl group-substituted transition metal complexes. Whereas transition metal complexes containing one or more halogen groups exhibit good shelf life, the corresponding alkyl-substituted transition metal complexes rapidly lose activity particularly in the presence of trace impurities.
Additionally, and presumably to some extent related to the instability of the alkyl-substituted transition metal complexes, preparative procedures employed to produce the cationic transition metal catalyst can be involved, often requiring critical mixing steps and specific reagents. In this regard, reference may be had to published European patent application 0500944A1 wherein a catalyst system is obtained by reacting a halogenated metallocene compound containing cyclopentadienyl or cyclopentadienyl derivative ligands with an organometallic compound and then bringing the resultant reaction product into contact with a compound which forms the active cationic catalyst. The reference discloses that the halogenated metallocene compound must first be reacted with the organometallic compound and the resultant product then brought into contact with the compound forming the active cationic catalyst. It goes on to state that if the order is wrong, the resulting catalyst systems do not polymerize xcex1-olefins at all or activity of the catalyst system is very low and reproducibility of polymerization is poor. Furthermore, the type of organometallic compound used is critical since, in the examples, it is shown that trimethyl aluminum does not react with the halogenated metallocene compound.
U.S. Pat. No. 5,817,725 similarly discloses that it is critical to first combine the neutral metallocene and alkylating agent before contacting with the ionizing agent in order to produce effective cationic metallocene catalysts. In Example 3 of the patent, it is shown that when the neutral metallocene is first reacted with the ionizing agent and then subsequently brought into contact with the trialkylaluminum, little or no polymerization was obtained.
Bergemann, et al., in the Journal of Molecular Catalysis A: Chemical 135 (1998), 41-45, also describe a procedure wherein a metallocene is dissolved in toluene and contacted with triisobutylaluminum before contacting with the cationic activator compound.
Active metallocene catalysts comprising a metallocene compound, an ionizing ionic compound, an organoaluminum compound and a Lewis base compound are described in U.S. Pat. No. 5,576,259. While patentees state that the method of catalyst preparation is not limited, all of the exemplified catalysts are made by first contacting the alkylaluminum with the metallocene compound and then, after mixing for a period of time, contacting with the ionizing ionic compound and Lewis base. When the Lewis base was omitted, significantly reduced yields were reported.
Published International application WO95/10546 discloses the preparation of cationic metallocene catalysts using alkylation agents which are a mixture or reaction product of an alkyl aluminum compound and an alcohol. Without alcohol, there was little or no activity. While the reference indicates that the order of combining the catalyst components is generally not critical, it goes on to state that best results are obtained when the co-catalyst, i.e., ionizing compound, is introduced into the reactor only after the other components of the catalyst system. Moreover, all of the examples in the application react the aluminum-containing alkylating agent with the metallocene before contacting with the ionizing cocatalyst compound.
In U.S. Pat. Nos. 5,519,100 and 5,614,457 metallocene catalysts formed from a neutral alkylated metallocene compound and ionic ionizing compound are combined with a mixture of the xcex1-olefin and aluminum alkyl to effect polymerization. In this process the metallocene is pre-alkylated before contact with the olefin/aluminum alkyl mixture. In all of the above references, the polymerization-stable bulky anionic ligands (also referred to as ancillary ligands) associated with the transition metal are carbocyclic ligands, such as cyclopentadienyl (Cp) or substituted Cp, indenyl, fluorenyl, etc. There is no disclosure or suggestion that anything but metallocenes with Cp or Cp type ancillary ligands can be employed in the reference processes. Furthermore, those references which disclose supported cationic metallocenes either require that the alkylating agent and metallocene be reacted as the first step of the procedure or disclose in general terms that the order of mixing can be varied. None of the prior art references show that effective and, in some instances, markedly superior catalysts can be produced when a metallocene and ionizing agent are first combined, with or without a support, and then in a later step contacted in the polymerization reaction system with an alkylating agent to form the active catalyst in-situ.
The present invention wherein a transition coordination metal complex containing one or more labile ligands, such as halogen, and an ionizing agent are contacted in a first step and subsequently contacted with an alkylating agent in the polymerization reaction system overcomes problems associated with the heretofore reported procedures. The process of the invention also eliminates the need for Lewis bases and alcohols which are necessary for some of the prior art procedures. Also, by judicious selection of the transition metal complex and/or alkylation agents, it is possible to vary polymerization rates, comonomer incorporation and resin properties. Furthermore, when the transition metal complex is deposited on certain pretreated supports, in some instances the resulting catalysts prepared using the in-situ process of the invention unexpectedly exhibit activity greater than that of the corresponding unsupported catalyst. In another embodiment of the invention it has unexpectedly been found that when a supported precatalyst is formed using a neutral transition metal complex having one or more ancillary ligands which are anionic heterocyclic ligands, proportionally higher activity and the ability to vary resin properties over a wider range is observed compared to systems where only carbocyclic anionic ligands are associated with the transition metal.
The invention relates a process for the formation of single-site transition metal olefin polymerization catalysts in the polymerization system and to polymerizations conducted therewith. The formation of the active cationic transition metal catalyst in-situ involves contacting a precatalyst and organometallic alkylating agent in the polymerization system. The precatalyst, which is formed outside the polymerization system environment, is obtained by contacting a boron-containing ionizing agent and a neutral transition metal coordination complex. The precatalyst, which can be either supported or unsupported, contains at least one labile ligand capable of being removed and replaced with an alkyl group when contacted with an organometallic alkylating agent. Typically the molar ratio of boron to transition metal for the precatalyst will be from 0.1:1 to 10:1. The precatalyst is then introduced into the polymerization reaction system and contacted with an organometallic alkylating agent and the active cationic transition metal catalyst formed in-situ. Initiation of the in-situ catalyst formation may be done in the presence or absence of the olefin(s) to be polymerized. In one embodiment of the invention, contact of the precatalyst and alkylating agent is carried out in an inert hydrocarbon medium and under polymerization conditions. The molar ratio of alkylating agent metal to transition metal used generally ranges from 1:1 to 1000:1.